Motor shaft, motor and motor assembly

ABSTRACT

A motor shaft is configured to be press-fitted into a hole formed in a press-fitted component from a front end side in an axial direction. The motor shaft includes: a shaft body; a first annular projection portion that protrudes outwardly in a radial direction of the shaft body and is brought into contact with an inner circumferential surface of the hole the first annular projection portion being positioned at a first position; and a second annular projection portion that protrudes outwardly in the radial direction of the shaft body and is brought into contact with the inner circumferential surface of the hole, the second annular projection portion being positioned at a second position that is shifted from the first position toward a rear end side in the axial direction of the shaft body.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a motor shaft, a motor and a motorassembly.

2. Description of the Related Art

A method of fixing a press-fitted component such as a gear to a motorshaft includes a method of forming a hole for inserting a motor shaftinto a press-fitted component, processing the hole such that the innerdiameter of the hole of the press-fitted component is slightly less thanthe outer diameter of the motor shaft and press-fitting and fixing themotor shaft into the hole of the press-fitted component.

Power required to press-fit the motor shaft in the press-fittedcomponent is approximately proportional to a value obtained bymultiplying a press-fitting interference ((the outer diameter of themotor shaft)−(the inner diameter of the press-fitted component), by apress-fitting length. If power required for press-fitting is greaterthan the allowable compression stress of the motor shaft, when the motorshaft is press-fitted into the press-fitted component, the motor shaftmay buckle. By decreasing the press-fitting length, power required forpress-fitting may decrease such that the motor shaft does not buckle.However, when the press-fitting length decreases, when the motor shaftis press-fitted into the press-fitted component, an inclination easilyoccurs in the axis of the motor shaft and the axis of the press-fittedcomponent. Therefore, it is not easy to fix the press-fitted componentto the motor shaft with high precision.

When the press-fitting length is large, it is possible to fix thepress-fitted component to the motor shaft with high precision. When thepress-fitting length is high, it is necessary to decrease the upperlimit of the press-fitting interference such that power required forpress-fitting is less than the allowable compression stress of the motorshaft. The tolerance of the outer diameter of the motor shaft and theinner diameter of the hole of the press-fitted component may be decidedaccording to design so as to decrease the upper limit of thepress-fitting interference. However, in an actual manufacturing process,it may be difficult to control the outer diameter of the motor shaft andthe inner diameter of the press-fitted component according to design.Therefore, adjustment in a manufacturing process such as stratifying ofthe processed motor shaft and press-fitted component is required suchthat power required for press-fitting becomes less than the allowablecompression stress of the motor shaft. Accordingly, there is a need fora motor shaft which may be press-fitted into a press-fitted componentwith small power such that the motor shaft does not buckle.

A serration-processed motor shaft capable of being press-fitted into afemale part with small power is suggested in JP-A-2012-257389.

However, even in the motor shaft suggested in JP-A-2012-257389, powerrequired for press-fitting increases in proportion to the press-fittinglength. When the press-fitting length increases, adjustment in amanufacturing process such as stratifying of the processed motor shaftand press-fitted component is required such that power required forpress-fitting becomes less than the allowable compression stress of themotor shaft.

SUMMARY OF THE INVENTION

One of objects of the present invention is to provide a motor shaftcapable of being press-fitted into a press-fitted component with smallpower to fix the press-fitted component with high precision, and a motorand motor assembly including the motor shaft.

According to an illustrative embodiment of the present invention, thereis provided a motor shaft that is to be press-fitted into a hole formedin a press-fitted component from a front end side in an axial direction.The motor shaft includes: a shaft body; a first annular projectionportion that protrudes outwardly in a radial direction of the shaft bodyand is brought into contact with an inner circumferential surface of thehole the first annular projection portion being positioned at a firstposition; and a second annular projection portion that protrudesoutwardly in the radial direction of the shaft body and is brought intocontact with the inner circumferential surface of the hole, the secondannular projection portion being positioned at a second position that isshifted from the first position toward a rear end side in the axialdirection of the shaft body.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional view showing a motor assembly according to afirst embodiment of the present invention;

FIG. 2 is a schematic view showing a motor shaft according to the firstembodiment of the present invention, wherein section (A) in FIG. 2 is aside view of the motor shaft, and section (B) in FIG. 2 is a front viewof the motor shaft;

FIG. 3 is a schematic view showing a press-fitted component into whichthe motor shaft according to the first embodiment of the presentinvention is press-fitted, wherein section (A) in FIG. 3 is a front viewof the press-fitted component, and section (B) in FIG. 3 is across-section view of the press-fitted component taken at A-A′ lineshown in section (A);

FIG. 4 is a diagram showing a relation between a load and apress-fitting amount when the motor shaft according to the firstembodiment of the present invention is press-fitted into thepress-fitted component;

FIG. 5 is a diagram showing a relation between a load and apress-fitting amount when a conventional motor shaft is press-fittedinto a press-fitted component;

FIG. 6 is a schematic view showing a motor shaft according to a secondembodiment of the present invention, wherein section (A) is a side viewof the motor shaft, and section (B) is a front view of the motor shaft;

FIG. 7 is a cross-sectional view showing a state in which the motorshaft according to the second embodiment of the present invention ispress-fitted into the press-fitted component;

FIG. 8 is a diagram showing a relation between a load and apress-fitting amount when the motor shaft according to the secondembodiment of the present invention is press-fitted into thepress-fitted component;

FIG. 9 is a cross-sectional view showing a state in which a motor shaftaccording to a first modification of the present invention ispress-fitted into a magnet;

FIG. 10 is a side view of a motor shaft according to a secondmodification of the present invention; and

FIG. 11 is a cross-sectional view showing a state in which aconventional motor shaft is press-fitted into a press-fitted component.

DETAILED DESCRIPTION

Hereinafter, the motor shaft, the motor and the motor assembly accordingto the embodiments of the present invention will be described withreference to the drawings. The same or similar portions of the drawingsare denoted by the same reference numerals.

FIG. 1 is a cross-sectional view of a motor assembly according to afirst embodiment of the present invention. As shown in FIG. 1, the motorassembly 100 includes a motor 101 including a motor shaft 1 and apress-fitted component 4. The motor 101 includes a magnet 12 mounted inthe motor shaft 1, a set of bearings 13 a and 13 b rotatably supportingthe motor shaft 1, a coil 14 and a case 11 for receiving the portions,in addition to the motor shaft 1.

The case 11 is a rectangular parallelepiped hollow member. Circularopenings are formed in the case 11 in an axial direction at the left andright sides of FIG. 1. The motor shaft 1 is rotatably supported by theset of bearings 13 a and 13 b in a state in which a front end thereofprotrudes outwardly from the right opening.

The magnet 12 is a cylindrical member having a through-hole throughwhich the motor shaft 1 penetrates. The magnet 12 is fixed to the motorshaft 1 inserted into the through-hole.

The coil 14 is arranged to surround the magnet 12. The coil 14 rotatesthe motor shaft 1 along with the magnet 12 by electromagneticinteraction with the magnet 12.

The press-fitted component 4 is fixed to the front end of the motorshaft 1. The motor shaft 1 has an annular projection 2 in a portion forfixing the press-fitted component 4 and is rotatably shaft-supported bythe bearings 13 a and 13 b and is provided in the case 11. The coil 14for generating a magnetic field is fixed to the case 11.

When current is supplied to the motor 101, the coil 14 generates arotation magnetic field. By this rotation magnetic field, the magnet 12receives power to rotate the motor shaft 1. Next, the motor shaft 1 ofthe first embodiment will be described.

As shown in FIG. 2, sections (A) and (B), the motor shaft 1 of the firstembodiment includes a cylindrical shaft body 10 having a diameter a andannular projections 2 which are formed at four places in an axialdirection and are brought into contact with the inner circumferentialsurface of a press-fitting hole 5 when the motor shaft 1 is press-fittedinto the press-fitted component 4.

The annular projections 2 are projections which protrude outwardly in aradial direction of the shaft body 10 and go around the outercircumferential surface. Two first annular projections 2 a areadjacently formed in the vicinity of the axial-direction front end ofthe press-fitting surface with the press-fitting hole 5 of thepressed-in shaft body 10. The two first annular projections 2 aconstitute a first annular projection group 3 a. The distance betweenthe adjacent first annular projections 2 a is X. Two second annularprojections 2 b are adjacently formed in the vicinity of theaxis-direction rear end of the press-fitting surface with thepress-fitting hole 5 of the pressed-in shaft body 10. The two secondannular projections 2 b constitute a second annular projection group 3b. The distance between the adjacent second annular projections 2 b isX. The first annular projection group 3 a and the second annularprojection group 3 b are formed to be separated from each other in theaxial direction. The distance between the first annular projection group3 a and the second annular projection group 3 b is Y which is greaterthan the distance X. The outer diameter of the first annular projections2 a is b and the outer diameter of the second annular projections 2 b isc which is greater than the outer diameter b. The axial-direction lengthof the first annular projection 2 a and the second annular projection 2b is d.

As shown in FIG. 2, sections (A) and (B), the press-fitted component 4includes a press-fitting hole 5 including a first press-fitting hole 5 awhich has a press-fitting length L, is formed in the vicinity of anentrance side in which the motor shaft 1 is press-fitted, has an innercircumferential brought into contact with the second annular projections2 b while the second annular projections bites and has a diameter C anda second press-fitting hole 5 b which is located at the inner side ofthe first press-fitting hole 5 a, has an inner circumferential surfacebrought into contact with the first annular projections 2 a while thesecond annular projections bites and has a diameter B less than thediameter C. The press-fitted component 4 is a part which is fixed to themotor shaft 1 and rotates along with the motor shaft 1 and isparticularly a gear or the like. The axial-direction length of the firstpress-fitting hole 5 a is E. The length L is greater than the distanceY. The diameter B is slightly less than the outer diameter b of theannular projections 2. The press-fitting interference is expressed by(the outer diameter b−the diameter B). When the diameter B is set to 1such that the press-fitting interference becomes an appropriate value,the outer diameter b is preferably 1.001 to 1.020. The diameter C isslightly less than the outer diameter c of the annular projections 2.The press-fitting interference is expressed by (the outer diameter c−thediameter C). When the diameter C is set to 1 such that the press-fittinginterference becomes an appropriate value, the outer diameter c ispreferably 1.001 to 1.020. The diameter B is greater than orsubstantially equal to the diameter a. The diameter B is preferablysubstantially equal to the diameter a so as to prevent idle rotation ofthe motor shaft 1 and the press-fitted component 4 due to contactbetween the portion other than the annular projections 2 of the motorshaft 1 and the inner circumferential surface of the press-fitting hole5. In order to reduce power required for press-fitting, the diameter Cis preferably greater than the outer diameter b.

Next, a method of press-fitting the motor shaft 1 into the press-fittedcomponent 4 will be described.

The press-fitted component 4 is fixed using a jig or the like. Alignmentis performed such that the axis of the press-fitting hole 5 of the fixedpress-fitted component 4 and the axis of the motor shaft 1 become equal.The motor shaft 1 is moved in the axial direction and the motor shaft 1is press-fitted into the press-fitting hole 5 from the side of the firstpress-fitting hole 5 a. When the motor shaft 1 is press-fitted up to apredetermined position, fixing of the motor shaft 1 and the press-fittedcomponent 4 is finished.

Next, a relation between a press-fitting amount and power required forpress-fitting (load for press-fitting) when the motor shaft 1 ispress-fitted into the press-fitted component 4 will be described. Thedistance Y is less than the axial-direction length E of the firstpress-fitting hole 5 a. A graph indicating the relation between thepress-fitting amount and the load when the motor shaft 1 is press-fittedinto the press-fitting hole 5 in arbitrary units is shown in FIG. 4.When the motor shaft 1 is press-fitted into the press-fitting hole 5,the second annular projections 2 b of the front end side of the secondannular projection group 3 b and the end of the first press-fitting hole5 a are brought into contact with each other such that the load forpress-fitting increases. While the end of the first press-fitting hole 5a passes through the second annular projections 2 b, the load becomes apeak. When the motor shaft is further press-fitted, the load decreases,but, when the end of the first press-fitting hole 5 a starts to bebrought into contact with the second annular projections 2 b of the rearend side of the second annular projection group 3 b, the load increasesagain. Therefore, when the end of the first press-fitting hole 5 apasses through the second annular projections 2 b of the rear end side,the load becomes a peak. Thereafter, the load for press-fittingdecreases. Next, when the motor shaft 1 is further press-fitted into thepress-fitting hole 5, the first annular projection 2 a of the front endside of the first annular projection group 3 a and the end of the secondpress-fitting hole 5 b are brought into contact with each other suchthat the load for press-fitting increases. While the end of the secondpress-fitting hole 5 b passes through the first annular projection 2 aof the front end side, the load becomes a peak. When the motor shaft isfurther press-fitted, the load decreases, but, when the end of thesecond press-fitting hole 5 b starts to be brought into contact with thefirst annular projections 2 a of the rear end side of the first annularprojection group 3 a, the load increases again. Therefore, when the endof the second press-fitting hole 5 b passes through the first annularprojections 2 a of the rear end side, the load becomes a peak.Thereinafter, when the motor shaft is further press-fitted, the load forpress-fitting decreases. The press-fitting width of the previous twopeaks and the press-fitting width of the next two peaks are equal to thedistance X. The widths of the previous two peaks and the next two peaksare less than the distance Y.

A relation between a press-fitting amount and a load when a conventionalmotor shaft S without the annular projections 2 shown in FIG. 11 ispress-fitted into the press-fitted component 4 will now be described. Agraph showing the relation between a press-fitting amount and the loadwhen the motor shaft S is press-fitted into the press-fitted component 4in arbitrary units is shown in FIG. 5. When the front end of the motorshaft S and the end of the press-fitting hole 5 start to be brought intocontact with each other, the load increases and becomes a peak.Thereafter, the load decreases and then linearly increases to become apeak when the motor shaft S is completely press-fitted into thepress-fitting hole 5. When the motor shaft S is further press-fittedinto the press-fitted component 4, the maximum load is required. Whenthe conventional motor shaft S is press-fitted into the press-fittedcomponent 4, the press-fitting distance requiring the load is a lengthL.

Next, power P required to press-fit the motor shaft 1 in thepress-fitted component 4 will be compared with power p required topress-fit the motor shaft S in the press-fitted component 4. Powerrequired for press-fitting is substantially proportional to ((thepress-fitting interference)×(the axial-direction length of contactbetween the motor shaft and the press-fitted component)). Power Pnecessary to press-fit the motor shaft 1 into the press-fitted component4 is substantially proportional to ((the press-fittinginterference)×((the axial-direction length d of contact between themotor shaft 1 and the press-fitted component 4)×(the number of annularprojections 2, which is 4 in this embodiment))). In contrast, power prequired to press-fit the motor shaft S in the press-fitted component 4is substantially proportional to ((the press-fitting interference)×(theaxial-direction length L of the press-fitting hole 5 of the press-fittedcomponent 4)). The outer diameter of the motor shaft S is e, the outerdiameter e is slightly greater than the diameter B, and thepress-fitting interference is expressed by (the outer diameter e−thediameter B).

As described above, according to the first embodiment, by forming, inthe shaft body 10, the annular projections 2, which are brought intocontact with the inner circumferential surface of the press-fitting hole5 of the press-fitted component 4, power required for press-fittingincreases in every annular projection 2 but decreases in portionsexcluding the annular projections 2. By setting the dimensions (Y and E)of the motor shaft 1 and the press-fitted component 4 such that loadoccurrence timings are deviated upon press-fitting, the press-fittingdistance necessary for press-fitting may decrease. In addition, whenpower P required for press-fitting is ((length d×4)/length L) timespower p required for press-fitting when (the press-fitting interferenceouter diameter b−the diameter B), (the outer diameter c−the diameter C)and (the outer diameter e−the diameter B) are the same. Morespecifically, when the length d =the length L/40, power P required forpress-fitting is 1/10 of power p required for press-fitting. Therefore,the motor shaft 1 may be press-fitted into the press-fitted componentwith a smaller load than that of the motor shaft S. That is, the maximumvalue of the load for press-fitting the motor shaft 1 shown in FIG. 4into the press-fitted component 4 is less than the maximum value of theload for press-fitting the motor shaft S shown in FIG. 5 into thepress-fitted component 4. Power P necessary for press-fitting may beadjusted by changing the length d even when the press-fittinginterference is the same. When the allowable compression stress of themotor shaft 1 and the allowable compression stress of the motor shaft Sare the same, the upper limit of the press-fitting interference in whichthe motor shaft 1 does not buckle is (length L/(length d×4)) times theupper limit of the press-fitting interference in which the motor shaft Sdoes not buckle. More specifically, when the length d=the length L/40,the upper limit of the press-fitting interference in which the motorshaft 1 does not buckle is 10 times the upper limit of the press-fittingtightening margin in which the motor shaft S does not buckle. By formingthe annular projections 2 in the motor shaft 1, the upper limit of thepress-fitting interference can increase, thereby reducing burdens onadjustment such as stratifying in the manufacturing process.

Since the outer diameter of the first annular projection 2 a is b andthe outer diameter of the second annular projection 2 b is c greaterthan the outer diameter b, when the motor shaft 1 is press-fitted intothe press-fitting hole 5, the motor shaft 1 may be press-fitted into thepress-fitted component 4 with low power until the first press-fittinghole 5 a and the second annular projection 2 b are brought into contactwith each other.

By adjacently forming the annular projections 2 and configuring theannular projection group 3, chipping of the front end of the projectionis left between the projections to increase biting. Since the number ofannular projections 2 may increase, collapse of the projections of theannular projections 2 or biting into the inner circumferential surfaceof the press-fitting hole 5 of the press-fitted component 4 may increaseto increase fastening power. Since a plurality of annular projections 2is mounted in the axis direction of the shaft body 10, it is possible toprevent the inclination of the axis of the motor shaft 1 and the axis ofthe press-fitted component 4. When the distance Y is large, it ispossible to further prevent the axial-direction inclination of thepress-fitted component 4. Therefore, the distance Y is preferably large.When the motor shaft 1 is made of a material having Vickers hardnessless than that of the press-fitted component 4, the annular projections2 collapse and galling with the inner circumferential surface of thepress-fitting hole 5 of the press-fitted component 4 is caused, suchthat the motor shaft is strongly fixed. For example, if the press-fittedcomponent 4 is sintered metal, the suitable material of the shaft isSUS303 of stainless steel.

In contrast, in the motor shaft S without the annular projections 2,power required for press-fitting linearly increases and does notdecrease in midstream and the press-fitting distance necessary forpress-fitting is the length L and cannot be reduced.

The motor shaft 1 of the second embodiment has a cylindrical shaft body10 having a diameter a and annular projections 2 which are formed at twoplaces of the axial direction and are brought into contact with theinner circumferential surface of the press-fitting hole 5 when the motorshaft 1 is press-fitted into the press-fitted component 4, as shown inFIG. 6, sections (A) and (B).

The annular projections 2 are projections which protrude outwardly in aradial direction of the shaft body 10 and go around the outercircumferential surface. A first annular projection 2 a is formed in thevicinity of the axial-direction front end of the press-fitting surfacewith the press-fitting hole 5 of the press-fitted shaft body 10. Asecond annular projection 2 b is formed in the vicinity of theaxial-direction rear end of the press-fitting surface with thepress-fitting hole 5 of the press-fitted shaft body 10. The outerdiameter of the first annular projection 2 a is b and theaxial-direction length thereof is d. The outer diameter of the secondannular projection 2 b is c and the axial-direction length thereof is d.The axis-direction distance between the first annular projection 2 a andthe second annular projection 2 b is Y.

The press-fitted component 4 is equal to that of the first embodiment.The method of press-fitting the motor shaft 1 into the press-fittedcomponent 4 is equal to that of the first embodiment. The motor shaft 1press-fitted into the press-fitted component 4 is shown in FIG. 7.

Next, a relation between a press-fitting amount and power required forpress-fitting (load for press-fitting) when the motor shaft 1 ispress-fitted into the press-fitted component 4 will be described. Thedistance Y is less than the axial-direction length E of the firstpress-fitting hole 5 a. A graph indicating the relation between thepress-fitting amount and the load when the motor shaft 1 is press-fittedinto the press-fitting hole 5 in arbitrary units is shown in FIG. 8.When the motor shaft 1 is press-fitted into the press-fitting hole 5,the second annular projection 2 b and the end of the first press-fittinghole 5 a start to be brought into contact with each other such that theload for press-fitting increases. While the end of the firstpress-fitting hole 5 a passes through the second annular projection 2,the load becomes a peak. Thereafter, the load decreases and, when thefirst annular projection 2 a and the end of the second press-fittinghole 5 b start to be brought into contact with each other, the loadincreases. When the end of the second press-fitting hole 5 b passesthrough the first annular projection 2 a, the load becomes a peak.Thereafter, the load decreases. The width of the press-fitting amount ofthe two peaks is less than the distance Y.

As described above, according to the second embodiment, by forming, inthe shaft body 10, the annular projection 2 which is brought intocontact with the inner circumferential surface of the press-fitting hole5 of the press-fitted component 4, power required for press-fittingincreases in the annular projection 2 but decreases in the portionsexcluding the annular projection. In contrast, in the motor shaft Swithout the annular projection 2, power required for press-fittinglinearly increases and does not decrease in midstream. When power Prequired to press-fit the motor shaft 1 into the press-fitted component4 is less than that of the motor shaft S without the annular projection2 if the press-fitting interference is the same similarly to the motorshaft 1 of the first embodiment. Therefore, similarly to the motor shaft1 of the first embodiment, the motor shaft 1 of the second embodimentmay be press-fitted into the press-fitted component 4 with a loadsmaller than that of the motor shaft S without the annular projection 2.The upper limit of the press-fitting interference in which the motorshaft 1 does not buckle may increase by forming the annular projection 2in the motor shaft 1 similarly to the motor shaft 1 of the firstembodiment, thereby reducing burdens on adjustment such as stratifyingin the manufacturing process.

Since a plurality of annular projections is formed in the axialdirection of the shaft body 10, it is possible to prevent theinclination of the axis of the motor shaft 1 and the axis of thepress-fitted component 4. Therefore, it is possible to fix the motorshaft 1 and the press-fitted component 4 with high precision. When thedistance Y is large, it is possible to further prevent theaxial-direction inclination. Therefore, the distance Y is preferablylarge. Since the outer diameter of the first annular projection 2 a is band the outer diameter of the second annular projection 2 b is c greaterthan the outer diameter b, when the motor shaft 1 is press-fitted intothe press-fitting hole 5, the motor shaft 1 may be pressed-fitted intothe press-fitted component 4 with low power until the firstpress-fitting hole 5 a and the second annular projection 2 b are broughtinto contact with each other. The motor shaft 1 of the second embodimentis suitable for cases in which there is a risk of bucking deformation ofa thin shaft.

When the motor shaft 1 is made of a material having Vickers hardnessless than that of the press-fitted component 4, the annular projection 2collapses and galling with the inner circumferential surface of thepress-fitting hole 5 of the press-fitted component 4 is caused, suchthat the motor shaft is strongly fixed. For example, if the press-fittedcomponent 4 is sintered metal, the suitable material of the shaft isSUS303 of stainless steel.

Although, in the first and second embodiments, the case in which theportion protruding from the case 11 of the motor shaft 1 is press-fittedinto the press-fitted component 4 is described, the press-fittedcomponent 4 may be located between two bearings supporting the shaft. Asshown in FIG. 9, the annular projection 2 may be formed in the motorshaft 1 of the portion, into which the magnet 12 is inserted. When themotor shaft 1 is press-fitted into the magnet 12, the outer diameter ofthe annular projection 2 of the front end side of the motor shaft 1 isless than that of the annular projection 2 of the rear end side, a metalsleeve 15 is loosely fitted into the through-hole of the magnet 12 andthe inner diameter of the entrance of the metal sleeve 15 is greaterthan that of the inner side of the metal sleeve. In this case, when themotor shaft 1 is press-fitted into the metal sleeve 15, the same effectsas when the motor shaft 1 is press-fitted into the press-fittedcomponent 4 is obtained. Since the plurality of annular projections isformed in the axial direction of the motor shaft 1, it is possible toprevent the inclination of the axes of the motor shaft 1 and the magnet12. The press-fitted component 4 may be a part having a weight, such asa flywheel made of tungsten and a part such as a metal hub for stoppingan impeller for a fan motor.

Although two annular projections 2 constitute the annular projectiongroup 3 in the first embodiment, three or more annular projections mayconstitute the annular projection group 3. Although two annularprojection groups 3 are formed in the motor shaft 1 in the firstembodiment, three or more annular projection groups may be formed in themotor shaft 1. Although two annular projections 2 are formed in themotor shaft 1 in the second embodiment, three or more annularprojections may be formed in the motor shaft 1. If the diameter e of themotor shaft S is small (6 mm or less), particularly, if power requiredfor press-fitting is large, the motor shaft S easily buckles. In themotor shaft 1 of the first and second embodiments, since power requiredfor press-fitting can decrease, it is possible to prevent the motorshaft 1 from buckling even when the diameter a of the motor shaft 1 issmall (6 mm or less).

Although the annular projection 2 has a rectangular cross section in thefirst and second embodiments, as shown in FIG. 10, the annularprojection 2 may have an arch cross section. In this case, the apex ofthe arch is elastically deformed such that the motor shaft 1 and thepress-fitted component 4 are fixed. Although the diameter b of the firstannular projection 2 a is less than the diameter c of the second annularprojection 2 b in the first and second embodiments, the diameter b maybe equal to the diameter c. In this case, since the press-fitting hole 5of the press-fitted component 4 may be made straight, it is easy tomanufacture the press-fitted component 4.

Various embodiments and modifications of the present invention arepossible without departing from the scope and spirit of the invention.The above-described embodiments have been disclosed for illustrativepurposes and do not limit the scope of the present invention.

As described with reference to the embodiments and its modifications,according to the present invention, there is provided a motor shaftcapable of being press-fitted into a press-fitted component with smallpower to fix the press-fitted component with high precision, and a motorand motor assembly including the motor shaft.

What is claimed is:
 1. A motor shaft that is press-fitted into a holeformed in a press-fitted component from a front end side in an axialdirection, the motor shaft comprising: a shaft body; a first annularprojection portion that protrudes outwardly in a radial direction of theshaft body and is brought into contact with an inner circumferentialsurface of the hole the first annular projection portion beingpositioned at a first position; and a second annular projection portionthat protrudes outwardly in the radial direction of the shaft body andis brought into contact with the inner circumferential surface of thehole, the second annular projection portion being positioned at a secondposition that is shifted from the first position toward a rear end sidein the axial direction of the shaft body.
 2. The motor shaft accordingto claim 1, wherein the first annular projection portion is separatedfrom the second annular projection portion.
 3. The motor shaft accordingto claim 2, wherein the first annular projection portion is positionedon a press-fitting surface of the shaft body where the press-fittedcomponent is inserted at the first position near the front end of theshaft body, and wherein the second annular projection portion ispositioned on the press-fitting surface near the rearmost end of thepress-fitting surface.
 4. The motor shaft according to claim 1, whereinan outer diameter of the first annular projection portion is smallerthan that of the second annular projection portion.
 5. The motor shaftaccording to claim 1, wherein the first annular projection portion isprovided with a plurality of first annular projections.
 6. The motorshaft according to claim 1, wherein the second annular projectionportion is provided with a plurality of second annular projections. 7.The motor shaft according to claim 1, wherein the motor shaft is made ofa material having hardness lower than that of the press-fittedcomponent.
 8. The motor shaft according to claim 7, wherein the motorshaft is made of SUS303 stainless steel.
 9. A motor comprising: themotor shaft according to claim 1; a magnet attached to the motor shaft;and a coil that rotates the motor shaft along with the magnet byelectromagnetic interaction with the magnet.
 10. A motor assemblycomprising: the motor according to claim 9; and the press-fittedcomponent.
 11. The motor assembly according to claim 10, wherein thehole of the press-fitted component includes: a first press-fitting holepositioned in the vicinity of an entrance into which the motor shaft ispress-fitted and having a first inner circumferential surface that isbrought into contact with the second annular projection portion in astate where the second annular projection portion bites into the firstinner circumferential surface; and a second press-fitting holepositioned at deeper side from the entrance with an inner diametersmaller than that of the first press-fitting hole and having a secondinner circumferential surface that is brought into contact with thefirst annular projection portion in a state where the first annularprojection portion bites into the second inner circumferential surface.12. The motor assembly according to claim 11, wherein a distance betweenthe first annular projection portion and the second annular projectionportion is smaller than a depth of the first press-fitting hole.